If a search character string including specific character data is input, files including the specific character data are narrowed down out of files to be searched by reference to index information indicative of which of multiple files to be searched includes the specific character data in a conventional technique. For example, refer to Japanese Patent No. 2986865.
Nonetheless, as the number of files to be searched increases, the volume of index information accordingly increases. The increase in the volume of the index information increases the time required for a process of referring to the index information, consequently causing a problem of an increase in the time consumed for a search process.